An active capacitive position detection system is known, which uses electrodes provided in a touch panel to detect a finger to additionally detect the presence and position of a stylus. A stylus used for this type of position detection system is called an “active stylus” and is configured such that a signal can be sent therefrom, via capacitive coupling with the electrodes, to a sensor controller provided in a position detection device. The sensor controller detects the presence and position of the stylus by detecting this signal.
As an example of such an active stylus, Patent Document 1 discloses a stylus configured to send to the sensor controller both a position signal used to derive coordinate data and a data signal representing information such as pen pressure value and unique stylus identifier (ID).
Patent Document 2 discloses another example of an active stylus. The stylus according to this example sends pen pressure detection results in a digital form. Also, in Patent Document 2, a position detection device includes a display device and a transparent sensor. The position detection device detects not only a position pointed to by a stylus and a pen pressure but also a position touched by a finger.
Numerous electronic apparatuses have come along that include a stylus configured to send signals using capacitive coupling. A plurality of signal transmission methods not compatible with each other have come into use as position detection systems included in these electronic apparatuses. Specifically, there exists a mixture of a method that uses a signal obtained by modulating a pulse train signal (including a pulse signal and a rectangular wave signal) as a signal sent by the stylus (hereinafter referred to as a “first method”) and a method that uses a signal obtained by modulating a sine wave signal (hereinafter referred to as a “second method”) as a signal sent by the stylus.
Patent Document 3 discloses an example of a position detection system that conforms to the first method. The stylus according to this example includes a pen pressure detector and a signal transmission section. The pen pressure detector optically detects a pen pressure. As illustrated in FIG. 10 of Patent Document 3, signals sent by the signal transmission section include a position signal pulse 910 used by the position detection device to detect a stylus position and a pressure signal pulse 950 indicating a pen pressure level detected by the pen pressure detector. The position signal pulse 910 is sent intermittently, and the pressure signal pulse 950 is sent in-between transmissions of the position signal pulses 910 only during detection of a pen pressure (when not in a hovering state). The position signal pulse 910 includes an alternating current (AC) pulse at a given frequency (specifically, 28.125 Hz), and the pressure signal pulse 950 includes a frequency-modulated pulse signal.
Patent Document 4 also discloses an example of a position detection system that conforms to the first method. The stylus according to this example includes a TX drive circuit 680 as illustrated in FIG. 9 of Patent Document 4, and the stylus is configured to send a TX signal 677, which is a pulse train signal similar to a TX signal 632 used during finger detection. The TX signal 677 is detected by a controller via capacitive coupling between the stylus and the sensor.
It should be noted that Patent Document 4 also discloses an arrangement for boosting during transmission of a signal by the stylus (paragraph 0084). According to this, a booster 870 is provided in the stylus to permit amplification of a TX signal 835 by a chip drive circuit 890 that is responsible for sending the TX signal 835.
In contrast, the position detection systems disclosed in Patent Documents 1 and 2 conform to the second method. A stylus according to the second method is configured to modulate a sine wave signal, rather than a pulse train signal such as a signal used during finger detection (signal sent from the sensor controller to the transmitting electrode inside the sensor), in accordance with the pen pressure level and so on, and send the modulated signal. Specifically, for example, the stylus according to the second method is configured to modulate a carrier signal at several hundred to several MHz by amplitude shift keying (ASK) or on-off-keying (OOK) and send the modulated signal. In order to extract the signal at the above frequency, the sensor controller receives the signal via a band-pass filter and recovers the pen pressure value.